It is known that chemical vapor deposition (CVD) serves to deposit metallic chemical species (Fe, Ru, Os, Co, Rh, Pd, Pt, etc. . . . ) in the form of nanoparticles on a support.
For this purpose, CVD makes use of organometallic precursors (e.g. metallic salts) of the chemical species, which precursors react at the surface of the substrate in order to deposit the chemical species in question.
The CVD technique can also be used to deposit complete catalytic systems on a support, by enabling chemical species such as oxides (Al2O3, CeO2, yttria-stabilized zirconia (YSZ), V2O5, BaO, TiO2, etc. . . . ) to be deposited in the form of continuous or discontinuous films on the support, followed by a step of depositing metallic species, as mentioned above.
Nevertheless, depositing such chemical species on spherical or similar surfaces forming a support gives rise to certain difficulties.
Thus, techniques have been proposed for improving the CVD method for depositing chemical species on such so-called spherical or similar surfaces.
To this end, a fluidized bed CVD technique has already been proposed.
Mention may be made for example of document FR 2 825 296 (D1) that discloses a device making use of a fluidized bed CVD technique. The device disclosed in D1 includes a sublimer with an organometallic precursor in solid form located therein, the sublimer being connected to a reservoir of vector gas such that the vector gas is capable of passing through the organometallic precursor in order to cause it to sublime (into the vapor phase). That device also has a column having support grains of spherical or similar shape located therein, said column being placed above the sublimer and being in leaktight fluid flow connection therewith so that the vapor of the organometallic precursor can be directed to the column. The device also has means for injecting an activation gas to activate the organometallic precursor injected within the sublimer in order to ensure that CVD takes place effectively on the support grains.
Nevertheless, that technique presents a certain number of drawbacks.
There is a risk of the support grains clogging the inlets of the fluidized bed zone.
There is also a risk of the deposited particles of the chemical species in question coalescing. Because injecting precursors into the fluidized bed zone does not necessarily ensure that the precursors are uniformly stirred within said zone, it can happen that deposition takes place in non-uniform manner on the support grains.
Furthermore, the rate of growth of the deposit is difficult to control in such devices. The operation that consists in subliming solid precursors is an operation that is difficult to control. Insofar as the surface areas of the solid precursors in contact with the gas stream passing therethrough varies over time (the shapes of the piles made up by the solid precursor changes, and it is also possible to find impurities or traces of moisture in the gas, thereby modifying the sublimable surface area), the amount of precursor that is sublimed at each instant is poorly controlled.
Furthermore, the sublimation surface area of the solid precursors present in the sublimer decreases over time as the precursors are consumed. Consequently, the rate of deposition of chemical species decreases over time, and the total duration of deposition turns out to be relatively lengthy, usually at least 30 minutes (min).